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5.8 Climatic Effects
Introduction
When working in large venues or outdoors we should always remember that sound
propagates through air and is affected by air temperature, humidity and wind.
The most audible of these effects is wind as it can vary dramatically in less than a
second causing rapidly swept filter effects that change middle and high frequency
content into incoherent noise.
Air temperature can change suddenly with very audible effects (eg when backstage
doors are opened during sound checks, venue doors are opened near the end of a
show in winter or cold air displaces the warm air trapped in a stadium during a clear
summer evening). Although quite rare, rapid air temperature changes can cause
sudden changes in propagation direction and major coverage problems for a few
fretful minutes before clearing. These sudden coverage changes often trigger sound
system investigations as they can sound like loudspeaker or amplifier failures.
Humidity tends to change slowly with time and affects the higher frequencies. This
slow change can be missed as our ear-brain systems tend to compensate for subtle
high frequency losses. If the relative humidity changes from, say, 25% at the
beginning of a hot afternoon's sound check to 40% as the weather turns sultry, we
may not notice the gradual 6dB increase in high frequency at the back of the field
(3dB at the mix position) until the guest engineer arrives, having walked the field
with a clean* pair of ears, and wants to change everything.
*Be aware that the human ear discharges more wax in humid conditions and this will
tend to negate the improved high frequency propagation.
All material © 2007. Martin Audio Ltd. Subject to change without notice.
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